hadoop LineRecordReader

/**
   *
   * @param job  配置信息
   * @param split  split
   * @param recordDelimiter   分列的字符
   * @throws IOException
   */
  public LineRecordReader(Configuration job, FileSplit split,
      byte[] recordDelimiter) throws IOException {
    //如果没有配置每一行的最大长度，那么每行默认的最大长度为Integer类型的最大值
    this.maxLineLength = job.getInt(org.apache.hadoop.mapreduce.lib.input.
      LineRecordReader.MAX_LINE_LENGTH, Integer.MAX_VALUE);
    start = split.getStart();
    end = start + split.getLength();
    final Path file = split.getPath();
    //默认io.compression.codecs  gzip
    compressionCodecs = new CompressionCodecFactory(job);
    codec = compressionCodecs.getCodec(file);

    // open the file and seek to the start of the split
    final FileSystem fs = file.getFileSystem(job);
    fileIn = fs.open(file);
    if (isCompressedInput()) {
      decompressor = CodecPool.getDecompressor(codec);
      if (codec instanceof SplittableCompressionCodec) {
        final SplitCompressionInputStream cIn =
          ((SplittableCompressionCodec)codec).createInputStream(
            fileIn, decompressor, start, end,
            SplittableCompressionCodec.READ_MODE.BYBLOCK);
        in = new CompressedSplitLineReader(cIn, job, recordDelimiter);
        start = cIn.getAdjustedStart();
        end = cIn.getAdjustedEnd();
        filePosition = cIn; // take pos from compressed stream
      } else {
        in = new SplitLineReader(codec.createInputStream(fileIn,
            decompressor), job, recordDelimiter);
        filePosition = fileIn;
      }
    } else {
      //从打开的文件找到指定位置
      fileIn.seek(start);
      in = new SplitLineReader(fileIn, job, recordDelimiter);
      filePosition = fileIn;
    }
    // If this is not the first split, we always throw away first record
    // because we always (except the last split) read one extra line in
    // next() method.
    //如果不是第一个dplit，那么将start设置成当前位置+第一行的长度。
    //跨过第一行，因为第一行数据已经在上一个split中读取了
    if (start != 0) {
      start += in.readLine(new Text(), 0, maxBytesToConsume(start));
    }
    this.pos = start;
  }